186 ❯ STEP 4. Review the Knowledge You Need to Score HighThe nerve cells can be divided into three main classes: sensory neurons, motor neurons,
and interneurons. Sensory neuronsreceive and communicate information from the sen-
sory environment. Interneuronsfunction to make synaptic connections with other neu-
rons. Located in the CNS, they tie together sensory input and motor output and are the
intermediaries of the operation. Motor neuronstake the commands of the CNS and put
them into action as motor outputs. This relationship is the basis for the reflex arc,which is
the basic unit of response in the CNS. A sensory neuron sends an impulse to the spinal
cord, which is transmitted via a series of interneurons to a motor neuron whose impulse
causes a muscular contraction.
Whoa! Did you see that spark zip past just now? That was a perfect example of a nerve
impulse. The membranes of these neurons all around us are full of pumps and special gated
ion channels that allow the cell to change its membrane potential in response to certain
stimuli. The opening of sodium channels causes the potential to become less negative, and
the cell is depolarized.If the threshold potential is reached (electrical potential that, when
reached, initiates an action potential), an action potential is triggered, which is the nerve
impulse that we just saw zip by. Action potentialsare quick changes in cell potential due to
well-controlled opening and closing of ion channels. The cell also contains potassium chan-
nels that open slowly in response to depolarization. After a short period of time, the sodium
channel closes, and potassium rushes out of the cell causing repolarizationof the cell and
lowering of the potential back down to its initial. Let’s move farther down this axon to see
where this impulse is going.
Here we are at the end of the axon, sometimes called the synaptic knob.This is where
calcium gates are opened in response to the changing potential, which causes vesicles to
release substances called neurotransmittersinto the synaptic gap between the axon and the
target cell. These neurotransmitters diffuse across the gap, causing a new impulse in the
target cell. Two of the most common neurotransmitters used in the body are acetylcholine
and norepinephrine. Substances called cholinesterases function to clear the neurotransmit-
ters from the synaptic gap after an action potential by binding to the neurotransmitters and
recycling them back to the neuron.
The ANS regulates involuntary activities in the body. As mentioned earlier, it is subdi-
vided into the parasympathetic and sympathetic divisions. For the most part, the para-
sympathetic response is one that promotes energy conservation: slower heart rate, decreased
blood pressure, and bronchial muscle and urinary bladder constriction. The sympathetic
response is one that prepares us for “fight or flight”—increased heart rate, dilated bronchial
muscles, increased blood pressure, and digestive slowdown.
The CNS consists of the brain and spinal cord. The brain is divided into various
sections that control the different regions of our bodies (Figure 15.5). The cerebellum
is in charge of coordination and balance. The medulla oblongatais the control center
for involuntary activities such as breathing. The hypothalamusis the thermostat andKEY IDEADendrites Myelin sheath surrounding axonCell bodyFigure 15.4 The components of a nerve cell (neuron).BIG IDEA 4.A.4
Organisms exhibit
complex properties
due to interactions
between constituent
parts.
BIG IDEA 3.D.2
Cells communicate
with each other
using chemical
signals (e.g.,
neurotransmitters).